How to Import Blender 2.79 built-in matcaps to Blender 2.80?
OP's comment here reminded me that you could probably check the v2.79 tag (i.e. release) to find these files.
Here they are on the official repository:
https://git.blender.org/gitweb/gitweb.cgi/blender.git/tree/v2.79:/release/datafiles/matcaps
Here they are on the GitHub mirror:
https://github.com/blender/blender/tree/v2.79/release/datafiles/matcaps
The one you're looking for looks like Matcap 9: